Field
The present invention relates to microelectromechanical devices and specifically to an inertial sensor with self-test capability and a self-test method for an inertial sensor.
Description of the Related Art
Micro-Electro-Mechanical Systems or MEMS can be defined as micro-scale mechanical and electro-mechanical systems where at least some elements have a mechanical functionality. MEMS structures can be applied to quickly and accurately detect very small changes in physical properties.
Capacitive microelectromechanical sensors have become part of many consumer devices and they are used also in a variety of safety critical applications, such as electronic stability control (ESC). Especially in the safety related applications, it is important to identify potential failures in mechanical or electrical signal paths of the capacitive sensor.
A capacitive sensor comprises at least one microelectromechanical element that comprises at least one capacitive element. The capacitive element comprises a rotor mass (a.k.a. proof mass or in short, just a rotor) and a stator which remains stationary while the rotor mass moves in response to acceleration. The position of the rotor mass in a reference system is measured by detecting signal capacitance. An electrode attached to or incorporated by the rotor mass and an electrode attached to or incorporated by the stator form a capacitance. When the rotor mass moves relative to the stator or inertial frame of reference, a change in the distance between the electrodes is converted to a change in the capacitance. A single variable capacitor is created between a static electrode of the stator and a moving electrode of the rotor mass (rotor). The total capacitance of the single variable capacitor includes a static capacitance defined by the capacitor configuration and a signal capacitance that results from the motion of the rotor mass in response to external acceleration. A capacitive element may comprise more than one variable capacitors, for example it can be formed as a capacitive bridge or a capacitive half-bridge.
Capacitive transducers in MEMS sensors often apply differential detection with two capacitors. For differential detection, in response to a detected activity, a first capacitor of a capacitor pair generates a first input signal, and a second capacitor of the capacitor pair generates a second input signal. The first input signal and the second input signal may be detected in parallel and processed in combination for added accuracy.
One example of a circuitry suitable for such differential detection is self-balancing capacitor bridge (SBB) where the capacitive transducer consists of a movable plate with a fixed electrode on each side. Together the three electrodes form two capacitors, the charges of which the SBB keeps balanced. Deflection of the plate is normalized with respect to the distance between the fixed electronics. The normalization of the self-balancing capacitor bridge provides a linear and stable transfer function, but higher signal-to-noise levels are required for many modern applications, especially with decreasing signal magnitudes attained from further miniaturized MEMS devices
Built-in functionality diagnostics is a way to ensure that a device may identify its own erroneous operation rapidly. Recognizing erroneous operation or failure of the device is especially important for devices which are used for critical functionalities. An example of devices with such critical functionality is accelerometers in automotive components. Continuous self-testing provides a reliable way to monitor the operation of a device, and provides significant amount of information of critical internal variations.
Multiplexing is a known method to decrease ASIC circuit area, when processing of more than one signal is needed. It allows using at least partially the same circuitry for processing multiple signals.
U.S. Pat. No. 6,629,448 presents a system where normal and self-test intervals alternate. When the device is in a self-test mode, self-test bias excitation and detection phases alternate. In self-test mode, a DC signal is used in biasing phase for excitation, and in detection phase the capacitance is detected utilizing an AC signal. The DC test signal cannot however be used during normal operation, since there is no way of making difference between self-test response and real inertial acceleration. In normal operation mode, the device uses zero bias DC voltage.
Patent application publication US 2009/0241634 presents a sensor system with continuous self-testing. Here a single test frequency with clearly higher frequency than the normal operation signal band is fed to the device, and a response is detected by a test response comparator in the output of the device during normal operation. This solution, however, requires wide band amplifiers and an additional demodulation stage, making the implementation power consuming and complex.